The present invention relates to the general field of controlling stages of variable-pitch vanes in a turbomachine.
In a turbomachine, it is known to use one or more stages of stator vanes for adjusting the flow direction and rate of gas passing through the compression section as a function of the operating speed of the turbomachine. Each such stator vane stage comprises a plurality of vanes (known as variable-pitch vanes) that can pivot about their respective pins connecting them to the stator so that their pitch angle can be modified as a function of the operating speed of the turbomachine.
Known devices for controlling a stage of variable-pitch vanes generally comprise a control member in the form of a ring surrounding the casing of the turbomachine, and a plurality of links or levers, with each link having a first end connected to the control ring via a hinge and a second end mounted on the pivot of a respective vane. A drive actuator is connected to the control ring in order to turn it about the axis of the turbomachine. When the ring turns about the turbomachine axis it causes all of the vanes of the stage to change their angular position synchronously.
When two axially-offset stages of variable-pitch vanes are to be controlled in synchronous manner, it is also known to make use of a synchronization bar to transmit the turning movement from the ring that is driven by the drive actuator to the control ring of the other stage. This transmission of movement takes place via bell cranks pivotally mounted on the casing of the turbomachine and connected firstly to the synchronization bar and secondly to respective ones of the control rings.
That control system generates movements in the various controlled stages that can be represented in the form of curves plotting the pitch angle of the vanes in the follower stage as a function of the pitch angle of the vanes in the leader stage. With a control system of the above-described type, such a curve, referred to as a “correlation” curve, can present a slope that varies, but only progressively. Thus, that type of control system can be used to command vane stages in simple manner only.
Aerodynamic requirements for controlling vane pitch are more and more frequently requiring control relationships that lead to correlation curves that include curves that present a sudden acceleration or deceleration of slope, particularly in their terminal portions.
Document EP 0 909 880 describes a variable-pitch device enabling non-linear control relationships to be obtained. In that device, each link of the leader stage is connected to the corresponding control ring by a connection having a slot and a stud sliding in the slot. Nevertheless, that control system is not fully satisfactory since it does not make it possible to reproduce specifically a correlation curve having a sudden acceleration or deceleration of slope.